This invention relates generally to free-piston Stirling engines, heat pumps and coolers and more particularly relates to improving the performance of a gamma configured free-piston Stirling machine with opposed power pistons by providing improved control of its output in a manner that can be more precisely adapted to and optimized for the operating conditions encountered by the Stirling machine. In the invention, a displacer has a connecting rod extending past the power pistons to an electromagnetic linear transducer. The linear transducer controls the amplitude and phase of the displacer's reciprocation allowing the linear transducer to control a Stirling cooler/heat pump in a manner that delivers a maximum rate of heat transfer or maximum efficiency over the entire range of operating temperatures and to control a Stirling engine in a manner that matches the power output of the engine to the load power demand while maximizing efficiency and stability over the entire range of operating temperatures and within the limits of the machine.
Fundamental Stirling Principles
As well known in the art, in a Stirling machine a working gas is confined in a working space that includes an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do mechanical work or to pump heat from the expansion space to the compression space. The working gas is cyclically shuttled between the compression space and the expansion space as a result of the motion of one or more power pistons and, in some machines a displacer. The compression space and the expansion space are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter. The shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and gas that is flowing into the expansion space through a first heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and gas that is flowing into the compression space through a second heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. In the embodiments of the invention that are illustrated in FIGS. 1-4, the first heat exchanger has the reference numeral 1 and the second heat exchanger has the reference numeral 2. The gas pressure is essentially the same in the entire work space at any instant of time because the expansion and compression spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work so it can function as an engine and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm heat sink.
As also well known in the art, there are three principal configurations of Stirling machines. The alpha configuration has at least two pistons in separate cylinders and the expansion space bounded by each piston is connected through a regenerator to a compression space bounded by another piston in another cylinder. These connections are arranged in a series loop connecting the expansion and compression spaces of multiple cylinders. The beta configuration has a single power piston, usually referred to simply as the piston, arranged within the same or a concentric cylinder as a displacer piston, usually referred to a simply a displacer. A gamma Stirling machine also has a displacer and at least one power piston but the piston is mounted in a separate cylinder alongside and sufficiently far from the axis of the displacer cylinder that the displacer and piston will not collide.
Stirling machines can operate in either of two modes to provide either: (1) an engine having its piston or pistons driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space and therefore capable of being a prime mover for a mechanical load, or (2) a heat pump having the power piston or pistons (and sometimes a displacer) cyclically driven by a prime mover for pumping heat from the expansion space to the compression space and therefore capable of pumping heat energy from a cooler mass to a warmer mass. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or for heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used generically to include both Stirling engines and Stirling heat pumps.
A Stirling machine that pumps heat from its expansion space is sometimes referred to as a cooler when its purpose is to cool a mass in thermal connection to its expansion space and sometimes is referred to as a heat pump when it purpose is to heat a mass in thermal connection to its compression space. They are fundamentally the same machine to which different terminology is applied Both “pump” (transfer) heat from an expansion space to a compression space. Working gas expansion in the expansion space absorbs heat from the interior walls surrounding the expansion space of the Stirling machine and working gas compression in the compression space rejects heat into the interior walls of the Stirling machine surrounding the compression space. Consequently, the terms cooler/heat pump, cooler and heat pump can be used equivalently when applied to fundamental machines.
Similarly a Stirling engine and a Stirling cooler/heat pump are basically the same power transducer structures capable of transducing power in either direction between two types of power, mechanical and thermal.
Problem to Which the Invention is Directed
As is well known, free-piston Stirling engines and coolers (FPSE/C) of the beta and gamma configurations employ two major moving parts, viz. the displacer and the piston or pistons as in opposed piston gamma configurations. The internally generated pressure variations of the working gas drives the displacer. This requires that the forces on the displacer be very carefully balanced so as to obtain the proper dynamic operation of the displacer. These forces consist of the spring forces, the inertia force, the pressure drop force and the differential pressure force across the displacer rod. The motion of the displacer directly controls the function of the machine, whether the machine is a cooler/heat pump, in which case the controlled function is the thermal lift, or the machine is an engine (prime mover), in which case the controlled function is the delivered mechanical power. The degree of lift or delivered power is determined by the relative phase angle between the displacer and piston motions and the amplitude of the motions of the displacer.
The essential problems and difficulties with driving the displacer with gas pressures alone are that:
a. In heat pumps, the maximum possible efficiency (or coefficient of performance) is not maintained at all operating conditions. The machine will therefore have increasingly compromised performance depending on how far the operating condition is from the design point.
b. In prime movers or engines, the problem is more severe in that it is often the case that stable operation with a changing load is only possible with an electronic controller between the load and the engine. This electronic controller needs a power capability at least as high as the maximum power delivered and a response time at least greater than the response time of the engine. There is also the problem of extracting the maximum efficiency at different operating conditions as in point (a).
It is therefore an object and feature of the invention to provide full but independent displacer control while minimizing added mass and dead volume in an opposed piston gamma configuration.
A further object of the invention is to provide an improved controllable free-piston Stirling configuration for opposed piston gamma type engines to control the displacer motions in order to change the power curve of the engine so that a variable but stable operating point is always established by assuring that the engine power curve grows with piston amplitude slower than the load curve does.
A further object of the invention is to provide an improved controllable free-piston Stirling configuration for opposed piston gamma type engines and heat pumps whereby the displacer motions are adjusted in order to maximize the efficiency or coefficient of performance depending on whether the device is operating as an engine or a heat pump.
A still further object of the invention is to provide an improved controllable free-piston Stirling configuration for opposed piston gamma type heat pumps in which the displacer phase may be reversed in order to pump heat in either direction through the machine.